why is not install ios 14 in iphone 6s

If you have an iPhone 6S you can install iOS 14.4.1.

If you have an iPhone 6 you can install iOS 12.5.1.

What version does your phone have?

You probably have a 6 and not a 6s.

Because you do not have a 6s. You have a 6 or a 6 Plus.

12.5.1 is as far as you can go. Your phone is obsolete.